Current and past methods of atmospheric leaching of primary metal sulfides (e.g., Chalcopyrite, Tennantite, and Enargite), may suffer from slow reaction kinetics and poor metal recoveries due to surface passivation effects during oxidative leaching. Surface passivation occurs when the growth of an elemental sulfur product layer occludes the surfaces of the particles being leached. The sulfur reaction product layer acts as a physical barrier, impeding the transport of reactants and products from the reaction plane.
A number of factors may enhance the detrimental effects of the sulfur product, with regard to metal dissolution, by altering the porosity and/or tortuosity of the product layer. These factors, individually or collectively, include crystal phase transformations, partial melting and recrystallization, or complete crystal melting. The range of passivation effects will depend upon the temperature of the reaction medium and the temperature at the reaction zone which may be different from the overall system temperature. This temperature difference may be sustained throughout the entire leach process or it may be transitory.
Other mechanisms of passivation can include the formation of non-stoichiometric, metal-deficient sulfide phases that are resistant toward further anodic dissolution reactions. Furthermore, if the dissolution of the metal sulfide is taking place via an electrochemical redox mechanism, the anodic dissolution step will be dependent upon the pH and redox potential at the reaction plane.
A number of factors, known to those skilled in the art, can make it difficult to maintain an optimum redox potential and thereby achieve complete metal recovery at maximum dissolution rates. In some instances, leaching of primary metal sulfides may also suffer from slow reaction kinetics and poor metal recoveries due to residual frothing agents used during froth flotation. The residual frothing agents may be present on particles being leached and interfere with superficial leaching chemistries.
A number of past methods have been attempted to increase metal leach rates by employing leach catalysts. One approach suggested addressing the passivation issue by increasing electron transport though an electrically-resistive, reaction-product layer by doping the layer with fine particulate carbon (see for example U.S. Pat. No. 4,343,773). Moreover, a more recently-proposed method (US-2012/0279357) for addressing passivation relies on the addition of an activated carbon catalyst to enhance the leach rate of arsenic-containing copper sulfides. Still other approaches have used silver-based catalytic leach systems for enhancing the copper dissolution rates in acidic ferric sulfate media (J. D. Miller, P. J. McDonough and P. J. Portillo, Electrochemistry in Silver Catalyzed Ferric Sulfate Leaching of Chalcopyrite, in Process and Fundamental Considerations of Selected Hydrometallurgical Systems, M. C. Kuhn, Ed., SME-AIME, New York, pp. 327-338, 1981), while others have used silver-activated pyrite to accomplish similar results (U.S. Pat. No. 8,795,612). The Applicant has further recently proposed a method and process for the enhanced leaching of copper-bearing sulfide minerals which utilizes microwave irradiation during leaching to combat the adverse effects of passivation on leaching (WO2014074985 A1).
Still others have adopted pre-leach ultra-fine grinding (i.e., mechanical activation) of a copper sulfide concentrate to achieve rapid post-grinding leach kinetics. U.S. Pat. No. 5,993,635 describes a method for recovering copper from sulfide-mineral compositions which comprises the step of ultra-fine grinding of the leach feed to a P80 of about 5 μm (see Example 3 in U.S. Pat. No. 5,993,635). While copper dissolutions of 95% or greater were achieved in 10 hours on a small scale, grinding to such a small particle size prior to leaching is not always economically in those cases where the leach feed is a low-grade metal concentrate.
Still others have combined ultra-fine grinding and leaching in so-called batch Mechano-Chemical leaching processes which are circular batch processes which do not provide for continuous downstream flow. Moreover, all prior art methods have required excessively large energy inputs to achieve significant levels of copper dissolution from chalcopyrite. While leach times to achieve 80% copper extraction have been demonstrated to be as short as 1 hour, the approach is difficult to adapt for large-scale commercial operation (D. A. Rice, J. R. Cobble, and D. R. Brooks, Effects of Turbo-milling Parameters on the Simultaneous Grinding and Ferric Sulfate Leaching of Chalcopyrite, RI 9351, US Bureau of Mines, 1991). Furthermore, copper recoveries in excess of 95-97% were not achievable due to passivation at high elemental sulfur loading.
As previously stated, the application and consumption of large specific energy renders the economic feasibility of full-scale industrial metal recovery in mechano-chemical processes impractical.